Are you talking about the ampersand or the percent symbol?the ampersand is highlighted because it triggers invalid.illegal.bad-ampersand.html scope in HTML.tmLanguage, using unescaped ampersand (also for querystring in links) should be avoided as it's formally an error in html.

So i centered the point in my previews answer.The & (ampersand), should be used escaped in html, even into link's querystrings, so it's better to write &amp; instead of the simple &.Although is commonly used unescaped, as a matter of fact it works in both ways, it's a formal error in html specs not to use the html entity for it, and ST is doing well highlighting it.This gives you 3 options: 1) just ignore it (like i do), 2) use &amp; in your code, 3) tweak/change html.tmlanguage